The success of meningococcal vaccine in stimulating serum bactericidal power against Neisseria meningitidis and in preventing meningococcal disease indicates that local and disseminated infections by another Neisseria, the gonococcus, may also be prevented by increasing bactericidal power. This approach is strengthened by the growing evidence that the serum bactericidal system can protect against gonorrhea and disseminated gonococcal infection. The object of this research will be to produce a vaccine capable of eliciting a borad spectrum of serum bactericidal activity against diverse strains of gonococci. The potential value of such a vaccine and of bactericidal antibody elicited by vaccination will be examined for its ability to kill gonococci in the milieu of the human genital tract. If protection is demonstrated by these experiments the use of vaccines for increasing serum bactericidal activity against the gonococcus should be considered in preventing gonorrhea and gonococcemia in man.